This invention relates to proportioning devices of the type used in fire-fighting apparatus for introducing foam producing liquid into the stream of water being pumped to the fire fighting location. Devices of this type are well known and are disclosed in U.S. Pat. No. 2,543,941.
The prior art devices comprise a positive displacement water motor coupled to a positive displacement foam liquid pump. The water motor is placed in a conduit through which water is discharged from a pressurized source and may consist of a rotary pump wherein intermeshing rotors are synchronized. The foam pump is placed in a conduit which bypasses the water motor and may consist of a gear type pump including two toothed rotors driven directly off of the water motor at the same RPM. In operation, flow through the fire line, which includes the conduit containing the water motor, drives the water motor causing the direct-connected foam liquid pump to inject a metered amount of foam liquid into the stream being fed to the fire-fighting location.
Typically, the foam pump and water motor are sized so that their combined discharge consists of 6% foam liquid and 94% water, with the percentage of foam liquid varying somewhat for different flow and inlet pressure conditions. The foam pump is a much smaller displacement device than the water motor to accommodate these conditions.
A problem with the prior art devices is that the slippage in the foam pump (ie., the leakage of liquid from the discharge of the foam pump back to its suction side) is a considerable amount. Moreover, the proportion of this leakage to the total flow is substantial.
It is the general object of this invention to improve foam liquid proportioners of the indicated type by reducing the proportion of the slippage to the total foam flow in the foam pump. To this end, the foam liquid proportioner of the invention is provided with a simple, self contained, gear step-up means for substantially increasing the RPM of the foam pump relative to the RPM of the water motor. By operating the foam pump at a high RPM, the total flow rate is increased and the slippage, which is a relatively fixed amount, is a smaller proportion of the total flow. In other words, the ratio of slippage to the total flow is reduced.
Since the foam liquid pump is substantially smaller in size than the water motor, it can be operated at much higher speeds than the water motor without any mechanical problems. Accordingly, the foam pump RPM can be increased to a ratio of 2 to 1 or 3 to 1 relative to the water motor without incurring any mechanical problems.